There is known a protective hood or helmet including protective windows (safety glasses) and contemplated to be used for performing welding, cutting and similar work (German Pat. No. 572,153), which helmet comprises a twopiece protective window the lower portion of which is made of an almost opaque material inhibiting or absorbing harmful radiation, and of an upper transparent viewing portion being formed as a grid adapted to be opened and closed. Opening and closing of the grid is effected by a small electromagnet or solenoid which is operated in response of the current passing through the welding or cutting torch.
It is also known (British Pat. No. 834,021) to provide a grid assembly comprising a stationary and a movable grid support each, whereby the movable support is adapted to be moved to and fro in front of the stationary support by an electromagnetic assembly against a spring bias, and adapted to be shifted by a specific distance in correspondence with the desired intensity of the light falling there through.
It has been found to be impossible to combine these conventional devices so as to to provide a universally useful protective device which is of light weight in operation and which may be manufactured at low cost. Especially, the conventional devices do not provide a cutting-off time lower than 1 ms. this short time rendering a real protection for the eyes. Generally, it is of disadvantage for the universal application that the protective device must be connected to the circuit of the welding apparatus. Particularly if a plurality of persons are present when a welding operation is carried out (training of apprentices!), it is not possible to connect all of their protective devices to the limited number of terminal jacks. Besides, so-called welding cabs are known where the operations are to be observed by third persons from the exterior. This construction, too, would require substantial technical expenditure in order to make the necessary connection. Furthermore, when the protective device is used in foundries and rolling mills, there is not present any electric circuit at all to which the protective device could be connected.
Accordingly, it is the object of the present invention to provide a protective device which may be employed in as universal a manner as possible for every work involving greatly varying intensities of light, which device should lend itself to inexpensive production and easy manufacture, and which device should be comfortable in use. Also, a particularly compact construction of the optical components should be possible, such that these components are easy to replace.
In a protective device of the kind as outlined at the beginning, these objects are solved in that photoelectric elements are provided adjacent said viewing window, said elements being adapted to produce a control signal equal to the intensity of light incident on said protective device and activating a control circuit for operating electromagnetic control elements. Accordingly, the invention departs from the principle of providing an exterior source for controlling the grid. Rather, the invention proposes to initiate the control in the region of the viewing window per se, whereby, for example, phototransistors, photocells and the like may be mounted to the edge of the window per se or immediately adjacent thereto at the protective device as such.
Furthermore, it is proposed to provide a logic circuit including a Darlington transistor pair or couple which in the presence of a control signal of the light-sensitive elements corresponding to a high intensity of light, supplies current to a first magnet coil, and which in the presence of a control signal corresponding to a low intensity of light drives a second magnet coil, whereby control elements for shifting the grid supports are drawn to and fro between the coils. This electro-mechanical connection allows to exert sufficient power for shifting the grids, whereby only a relatively small rate of movement is required in order to align the grids with each other. Preferably, spacers disposed between the grid supports are additionally provided, which spacers reduce the frictional drag and substantially fully eliminate wear of the grid strips by rubbing against each other. Also, it is suggested that the width of the grid strip surface, plus the viewing strips or bars, is less than 2 mm, such that a small amount of movement only is necessary. In order that the sensitivity of the optical-electronic portion of the protective device may easily and automatically be matched to the ambient light intensity, a light-sensitive series resistor is coupled to the logic control circuit, which resistor responds to the ambient light only and sets the sensitivity of the logic circuit to various levels.
Preferably, the magnet coils are provided with yokes of a magnetic material, which yokes exhibit remanence or residual magnetism upon the termination of the current flow therethrough, such that a control element for shifting the grid supports remains in the respective shifted position reached at that moment. By this structure, pulsating or intermittent operation becomes possible.
If integrated circuits and transistors will be used, which only consume energy during the switching action, for operation only a 9 V -- baby cell is necessary, which will last 6 months. Also, so called solar cells could be used, which get their energy by the welding light itself.
Furthermore, the invention proposes that one of the two grid supports is securely connected to an armature being positioned for movement between the magnetic portions of a pair of magnet coils. With a correspondingly small spacing between the two magnet coils, positive switching or shifting of the grids can be obtained even with a small current pulse. Advantageously, the magnetic yokes of the magnet coils are arranged in such a way that a pair of U-shaped yokes oppose each other with their free legs. An armature provided with a plug-in slot is disposed between the yokes, with a coupling member, preferably made of non-conductive plastic material and connected to one of the grid supports, being adapted to be inserted into said slot.
To extend the field of application, it is contemplated that the viewing window, comprising the pair of grid supports and a coupling member for connection to the armature, is mounted as a unit within a frame adapted to be slipped on, so as to be exchangeable in accordance with the respective varying working conditions.
Under all working conditions, it is expedient that the support material and the grid strips act to filter out the invisible radiation in the fringe area of the visible light spectrum, or that the grid strips with respect to their filter effect are adapted to the E.W. (electric welding) Standards.
Finally, the present invention provides a replaceable viewing window to be used in protective hoods for welding and for other protective or safety devices, comprising a first stationary grid support mounted in a frame, and a second grid support movable relative to said first grid support, whereby said second grid support is provided with a coupling member for connection to a movable armature, which armature is secured directly to said protective hood together with the electric components of a logic control circuit. In this construction, the viewing window or glass can be readily replaced should it become damaged or when the light conditions vary. Furthermore, the viewing window may be provided with filters for the photoelectric elements such that, when the viewing window is changed, the characteristic of these elements is altered, too.